That's what I am trying to figure out, but I haven't seen any progress on it.
Medieval 2 had an extra line called "stat_armour_ex" and here's how it looks:
Code:
stat_armour_ex armour value of first upgrade, armour value of second upgrade, armour value of third upgrade, armour value of fourth upgrade, defensive skill, melee shield value, ranged shield value (this would allow them to be invulnerable to missiles while weak in melee), sound type when hit
Unfortunately, everytime this code is added, the game CTD's upon startup. I've also tried implementing this line into the "stat_pri_armour" line, and while the game doesn't crash, the game only reads the first three values and then the sound. It ignores the rest.
I've tried this, but that only makes them tougher to kill in melee as well. The shield factor adds to armour only when the unit is attacked from the front or from the left. Seeing how the unit is being attacked from the front in melee, the shield value will be contributed, thus making the unit a lot harder to kill.